Emergencies Ministry spokesman Viktor Beltsov said two carriages went off the track about 150km from Moscow, causing injury to about 10 passengers - two of whom needed hospital treatment.
"The train driver said that the explosion took place in front of the engine," Beltsov said, adding that the bomb had left a one-metre deep crater.
Earlier reports had said some passengers heard a loud bang before the derailment.
Mainly Muslim Chechnya has been devastated by conflict for more than a decade, and fighting between Russian troops and Chechen separatist rebels are daily events.
Russia says it is stabilising the region but still maintains nearly 100,000 troops in Chechnya.